Ethylbenzene is a key raw material in the production of styrene and is produced by the reaction of ethylene and benzene in the presence of an acid catalyst. Old ethylbenzene production plants, typically built before 1980, used AlCl3 or BF3 as the acidic catalyst. Newer plants have in general been switching to zeolite-based acidic catalysts.
Traditionally, ethylbenzene has been produced in vapor-phase reactor systems, in which the ethylation reaction of benzene with ethylene is carried out at a temperature of about 380-420° C. and a pressure of 9-15 kg/cm2-g in multiple fixed beds of zeolite catalyst. Ethylene exothermally reacts with benzene to form ethylbenzene, although undesirable chain and side reactions also occur. About 15% of the ethylbenzene formed further reacts with ethylene to form di-ethylbenzene isomers (DEB), tri-ethylbenzene isomers (TEB) and heavier aromatic products. All these chain reaction products are commonly referred as polyethylated benzenes (PEBs). In addition to the ethylation reactions, the formation of xylene isomers as trace products occurs by side reactions. This xylene formation in vapor phase processes may yield an ethylbenzene product with about 0.05-0.20 wt % of xylenes. The xylenes show up as an impurity in the subsequent styrene product, and are generally considered undesirable.
In order to minimize the formation of PEBs, a stoichiometric excess of benzene, about 400-900% per pass, is applied, depending on process optimization. The effluent from the ethylation reactor contains about 70-85 wt % of unreacted benzene, about 12-20 wt % of ethylbenzene product and about 3-4 wt % of PEBs. To avoid a yield loss, the PEBs are converted back to ethylbenzene by transalkylation with additional benzene, normally in a separate transalkylation reactor.
By way of example, vapor phase ethylation of benzene over the crystalline aluminosilicate zeolite ZSM-5 is disclosed in U.S. Pat. Nos. 3,751,504 (Keown et al.), 3,751,506 (Burress), and 3,755,483 (Burress).
In recent years the trend in industry has been to shift away from vapor phase reactors to liquid phase reactors. Liquid phase reactors operate at a temperature of about 170-250° C., which is below the critical temperature of benzene (about 290° C.). One advantage of the liquid phase reactor is the very low formation of xylenes and other undesirable byproducts. The rate of the ethylation reaction is normally lower compared with the vapor phase, but the lower design temperature of the liquid phase reaction usually economically compensates for the negatives associated with the higher catalyst volume. Thus, due to the kinetics of the lower ethylation temperatures, resulting from the liquid phase catalyst, the rate of the chain reactions forming PEBs is considerably lower; namely, about 5-8% of the ethylbenzene is converted to PEBs in liquid phase reactions versus the 15-20% converted in vapor phase reactions. Hence the stoichiometric excess of benzene in liquid phase systems is typically 150-400%, compared with 400-900% in vapor phase.
Liquid phase ethylation of benzene using zeolite beta as the catalyst is disclosed in U.S. Pat. No. 4,891,458 and European Patent Publication Nos. 0432814 and 0629549. More recently it has been disclosed that MCM-22 and its structural analogues have utility in these alkylation/transalkylation reactions, see, for example, U.S. Pat. No. 4,992,606 (MCM-22), U.S. Pat. No. 5,258,565 (MCM-36), U.S. Pat. No. 5,371,310 (MCM-49), U.S. Pat. No. 5,453,554 (MCM-56), U.S. Pat. No. 5,149,894 (SSZ-25); U.S. Pat. No. 6,077,498 (ITQ-1); and U.S. Pat. No. 6,231,751 (ITQ-2).
Commercial liquid phase ethylbenzene manufacturing processes generally employ a plurality of series-connected alkylation reaction zones, each containing a bed of alkylation catalyst. Most, if not all, of the benzene is normally fed to a first inlet reaction zone, whereas the ethylene feed is typically divided substantially equally between the reaction zones. Poisons can and do enter the alkylation reaction system with both the ethylene and benzene feeds and the alkylation system frequently includes a by-passable reactive guard bed, which is normally located in a pre-reactor separate from the remainder of the alkylation system. The reactive guard bed is also loaded with alkylation catalyst and is maintained under ambient or up to alkylation conditions. Benzene and at least a portion of the ethylene are passed through the reactive guard bed prior to entry into the inlet zone of the series-connected alkylation reaction zones. The reactive guard bed not only serves to effect the desired alkylation reaction but is also used to remove any reactive impurities in the feeds, such as nitrogen compounds, which could otherwise poison the remainder of the alkylation catalyst.
By virtue of the poisons in the benzene and ethylene feeds, the catalyst in the reactive guard bed, or where there is no reactive guard bed, the catalyst in the inlet alkylation reaction zone, is subject to more rapid deactivation, and hence requires more frequent regeneration and/or replacement, than the remainder of alkylation catalyst. To reduce the cost and potential lost production time involved in this regeneration and/or replacement, there is significant interest in developing alkylation processes which maximize the cycle length of the catalyst in the reactive guard bed and/or the inlet alkylation reaction zone.
Although the preceding discussion has focused on the production of ethylbenzene, it will be appreciated that similar comments apply to the production of other alkylaromatic compounds, such as cumene and sec-butylbenzene, in which the alkylating group comprises other lower (C2-C5) alkenes, such as propylene and 1-butene and/or 2-butene.
The present disclosure provides an aromatics alkylation process that allows the use of a catalyst in the reactive guard bed or the inlet bed (first alkylation reaction zone) which exhibits an increased poison capacity (on a moles of poison per unit mass of catalyst basis), as a result of which the reactive guard bed or the inlet bed exhibits an increased cycle length between catalyst change-outs. This can be accomplished by providing an alkylation catalyst in the reactive guard bed or the inlet bed which has a greater amount of acid sites per unit mass of the catalyst than the alkylation catalyst in the second bed (second alkylation reaction zone).
U.S. Pat. No. 5,998,687 discloses a process for producing ethylbenzene comprising: a) contacting a first feed comprising benzene and ethylene with a first catalyst comprising zeolite beta in a first catalyst zone at first alkylation conditions to obtain a first effluent, and withdrawing the first effluent from the first catalyst zone at a first temperature; and b) contacting a second feed including at least a portion of the first effluent and comprising ethylene and benzene with a second catalyst comprising zeolite Y in a second catalyst zone at second alkylation conditions to obtain a second effluent comprising ethylbenzene, and withdrawing the second effluent from the second catalyst zone at a second temperature, wherein the second temperature is higher than the first temperature.
U.S. Pat. No. 6,057,485 discloses a process for producing ethylbenzene by gas-phase alkylation over a split load of monoclinic silicalite alkylation catalysts having different silica/alumina ratios. A feedstock containing benzene and ethylene is applied to a multi-stage alkylation reaction zone having a plurality of series-connected catalyst beds. At least one catalyst bed contains a first monoclinic silicalite catalyst having a silica/alumina ratio of at least 275 and at least one other catalyst bed contains a second monoclinic silicalite catalyst having a silica/alumina ratio of less than about 275. The alkylation reaction zone is operated at temperature and pressure conditions in which the benzene is in the gaseous phase to cause gas-phase alkylation of the aromatic substrate in the presence of the monoclinic silicalite catalysts to produce an alkylation product. The alkylation product is then withdrawn from the reaction zone for separation and recovery. The use of the split load of catalyst is said to allow a higher purity ethylbenzene product to be produced at improved efficiencies than if only one of the catalysts were used by itself.
U.S. Pat. No. 6,995,295 discloses a process for producing an alkylaromatic compound by reacting an alkylatable aromatic compound with a feed comprising an alkene and an alkane in a multistage reaction system comprising a plurality of series-connected alkylation reaction zones each containing an alkylation catalyst. The process comprises: (a) operating at least one of said alkylation reaction zones under conditions of temperature and pressure effective to cause alkylation of said aromatic compound with said alkene in the presence of said alkylation catalyst and to maintain said temperature and pressure being such that part of said aromatic compound is in the vapor phase and part is in the liquid phase; (b) withdrawing from said one alkylation reaction zone an effluent comprising said alkylaromatic compound, unreacted alkylatable aromatic compound, any unreacted alkene and said alkane; (c) removing at least part of said alkane from said one alkylation reaction zone effluent to produce an alkane-depleted effluent; and (d) supplying said alkane-depleted effluent to another of said alkylation reaction zones. The process may employ a by-passable reactive guard bed which is located in a prereactor separate from the remainder of the alkylation system and which is loaded with alkylation catalyst, which may be the same of different from the catalyst used in the alkylation reaction zones.